The tweeter upgrade changes the balance somehow, seems to allow the subs and mids to kick a little more while the tweeter handles most of the detail. Like I say you can turn the treble down which aids bass.
It's not a world class system after this, it's missing a proper amp after all but makes the cr@ppy stock system at least good. If you're a real audiophile then this won't cut it, you need Alpine at least or total aftermarket. IDK your personal taste so perhaps have a word with Musicar.